
Reset and clock control (RCC)
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The system clock maximum frequency is 80 MHz. After a system reset, the MSI oscillator, at
4 MHz, is selected as system clock. When a clock source is used directly or through the PLL
as a system clock, it is not possible to stop it.
A switch from one clock source to another occurs only if the target clock source is ready
(clock stable after startup delay or PLL locked). If a clock source which is not yet ready is
selected, the switch will occur when the clock source becomes ready. Status bits in the
Internal clock sources calibration register (RCC_ICSCR)
indicate which clock(s) is (are)
ready and which clock is currently used as a system clock.
6.2.9
Clock source frequency versus voltage scaling
The following table gives the different clock source frequencies depending on the product
voltage range.
6.2.10 Clock
security system (CSS)
Clock Security System can be activated by software. In this case, the clock detector is
enabled after the HSE oscillator startup delay, and disabled when this oscillator is stopped.
If a failure is detected on the HSE clock, the HSE oscillator is automatically disabled, a clock
failure event is sent to the break input of the advanced-control timers (TIM1/TIM8 and
TIM15/16/17) and an interrupt is generated to inform the software about the failure (Clock
Security System Interrupt CSSI), allowing the MCU to perform rescue operations. The CSSI
is linked to the Cortex
®
-M4 NMI (Non-Maskable Interrupt) exception vector.
Note:
Once the CSS is enabled and if the HSE clock fails, the CSS interrupt occurs and a NMI is
automatically generated. The NMI will be executed indefinitely unless the CSS interrupt
pending bit is cleared. As a consequence, in the NMI ISR user must clear the CSS interrupt
by setting the CSSC bit in the
Clock interrupt clear register (RCC_CICR)
.
If the HSE oscillator is used directly or indirectly as the system clock (indirectly means: it is
used as PLL input clock, and the PLL clock is used as system clock), a detected failure
causes a switch of the system clock to the MSI or the HSI16 oscillator depending on the
STOPWUCK configuration in the
Clock configuration register (RCC_CFGR)
, and the
disabling of the HSE oscillator. If the HSE clock (divided or not) is the clock entry of the PLL
used as system clock when the failure occurs, the PLL is disabled too.
Table 33. Clock source frequency
Product voltage
range
Clock frequency
MSI
HSI16
HSE
PLL/PLLSAI1/PLLSAI2
Range 1
(1)
1. Also for SMPS Range1 and SMPS Range2 High
48 MHz
16 MHz
48 MHz
80 MHz
(VCO max = 344 MHz)
Range 2
(2)
2. Also for SMPS Range2 Low
24 MHz range
16 MHz
26 MHz
26 MHz
(VCO max = 128 MHz)